Characterization of rabbit masseter muscle fibers

Molecular basis of the phenotypic characteristics of mammalian muscle fibres

Adult mammalian skeletal muscle fibres can be separated into two distinct groups, fast and slow. Within each group there is a continuum of metabolic enzyme activity levels. In addition there are fast and slow isoforms of various myofibrillar proteins such as myosin, tropomyosin and troponin. These proteins are multimeric and multiple isoforms of their subunits assemble to create a continuum of subtypes within each major group. Fibres which coexpress both fast and slow subunit isoforms have an increased number of possible isoform combinations such that an entire spectrum of fibre 'types' is found between the two extremes, fast and slow. Numerous myosin heavy chain and fast troponin T isoforms further increase the diversity of muscle fibres. Such cellular diversity helps to explain the dynamic nature of skeletal muscle. Each individual fibre is able to respond to various functional demands by appropriate changes in its phenotypic expression of specific proteins.

Jaw-closing muscles of kangaroos express alpha-cardiac myosin heavy chain

The masseter muscle of eutherian grazing mammals typically express beta or slow myosin heavy chain (MyHC). Myosins in the masseter of 4 species of kangaroos and a slow limb muscle of one of them were compared with their cardiac myosin by pyrophosphate and sodium dodecyl sulphate (SDS) gel electrophoresis, immunoblotting and immunohistochemistry. It was found that ventricular muscle contains three isoforms homologous to V1 (alpha-MyHC homodimer), V2 (heterodimer) and V3 (beta-MyHC homodimer) of eutherian cardiac muscle, and that the masseter contained V1, with traces of V2 and V3, in great contrast to eutherian ruminants, which express only V3. A polyclonal antibody (anti-KJM) was raised in rabbits against red kangaroo masseter myosin. After cross-absorption against limb muscle myofibrils, anti-KJM specifically reacted in Westerns with MyHCs from masseter but not limb muscles, and immunohistochemically with masseter, but not limb muscle fibers. In pyrophosphate Western blots, anti-KJM reacted with V1 but not with V3. However, a monoclonal antibody specific for eutherian slow myosin stained all kangaroo slow muscle fibers but only weakly stained scattered fibers in the masseter. The SDS-PAGE revealed that light chain composition of masseter and ventricular myosins is identical, but isoforms of both light chains of kangaroo limb slow myosin were observed. These results confirm that kangaroo jaw muscle express alpha-MyHC rather than beta-MyHC. The difference in MyHC gene expression between marsupial and eutherian grazers may be related to the fact that kangaroos are not ruminants, and have only a single chance to comminute food into fine particles, hence the need for the greater speed and power of muscle contraction associated with V1 containing muscle fibers.

Neuromuscular fatigue during repeated exhaustive submaximal static contractions of knee extensor muscles in endurance-trained, power-trained and untrained men

The neural and muscular changes during fatigue produced in repeated submaximal static contractions of knee extensors were measured. Three groups of differently adapted male subjects (power-trained, endurance-trained and untrained, 15 in each) performed the exercise that consisted of 10 trials of submaximal static contractions at the level of 40% of maximal voluntary contraction (MVC) force till exhaustion with the inter-trial rest intervals of 1 min. MVC force, reaction time and patellar reflex time components before and after the fatiguing exercise and following 5, 10 and 15 min of recovery were recorded. Endurance-trained athletes had a significantly longer holding times for all the 10 trials compared with power-trained athletes and untrained subjects. However, no significant differences in static endurance between power-trained athletes and untrained subjects were noted. The fatigue test significantly prolonged the time between onset of electrical and mechanical activity (electromechanical delay) in voluntary and reflex contractions. The electromechanical delay in voluntary contraction condition for power-trained and untrained subjects and in reflex condition for endurance-trained subjects had not recovered 15 min after cessation of exercise. No significant changes in the central component of visual reaction time (premotor time of MVC) and latency of patellar reflex were noted after fatiguing static exercise. It is concluded, that in this type of exercise the fatigue development may be largely owing to muscle contractile failure.

Function-dependent anatomical parameters of rabbit masseter motor units

Rabbit masseter motor units (22) were studied by stimulation of trigeminal motoneurons. We tested the hypotheses that masseter motor units facilitate fine motor control by concentrating fibers in small areas and that the distribution of motor unit fibers depends on the fiber type. The twitch contraction time and the isometric tetanic force were registered. The motor unit fibers were depleted of their glycogen by prolonged stimulation. Serial sections of the entire muscle were stained with the periodic acid Schiff (PAS) and monoclonal antibody stains. The muscle fibers of the motor unit were mapped and identified by four myosin heavy-chain (MHC) isoforms: I, IIA, IID, and cardiac-alpha. In the PAS-stained sections, anatomical parameters of the motor units, affecting the force output, were analyzed: the innervation ratio (IR), motor unit territory area (TA), and relative (R-DENS) and absolute (A-DENS) motor unit fiber densities. The fiber cross-sectional area (F-CSA) was measured for each MHC fiber type. The F-CSA sum of all motor unit fibers, the physiological cross-sectional area (P-CSA), was calculated. The IR ranged between 77 and 720 fibers (mean, 267). The mean TA was 8.71 mm2 (range, 4.45 to 19.58). The mean R-DENS was 10 fibers per 100; the A-DENS was 31 fibers per mm2. Linear correlations were found between the IR and the R-DENS and between the tetanic force and the IR. The F-CSAs showed a stepwise increase in value from type I- to IID-MHC fibers. The mean P-CSA was 0.90 mm2 (range, 0.09 to 2.97). A high linear correlation was noted between the P-CSA and the tetanic force. In conclusion, increase of motor unit size expressed in higher fiber counts and forces is accomplished by increase of the fiber density. Thus, forces can be exerted selectively in restricted regions of the masseter muscle. Differences in fiber orientation due to complex muscle pinnation emphasize the possibility of an accurate muscle performance.

Plasticity of craniomandibular muscle function: 31P magnetic resonance spectroscopy of the rabbit masseter muscle

The masseter muscle was studied during postnatal development of the rabbit from the juvenile to adult stage in which the oral function was altered during maturation by modifying the diet to soft food. The muscle was assessed using phosphate magnetic resonance (31P NMR) spectroscopy with a single-turn copper surface coil to study potential changes in phosphate metabolism. The 31P NMR spectra consisted of five peaks related to unbound forms of inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to the adenosine triphosphate (ATP). The masseter was assessed in one group of five rabbits at 8 weeks postnatally (juvenile) and after 4 months of this experimental group masticating on soft food. They were compared with a control group of five rabbits raised on a normal hard diet. The Pi/PCr ratio increased in the adult masseter much higher during twitching, tetany, and periodic contraction than in the juvenile regardless as to whether the adult animal had been raised from the juvenile period on soft or hard diet. There were relatively few differences between the experimental adult animals raised on a soft diet and the normal adult animals despite the soft diet animals demonstrating a significantly lower weight and smaller muscle mass. These findings suggest that chronic underuse of the masseter muscle by decreasing the masticatory loads has a minimal effect on the phosphate metabolism of the maturing masseter.

31P-magnetic resonance spectroscopy of the rabbit masseter muscle

Dynamic biochemical changes in the masseter muscle were studied in 14 New Zealand adult male rabbits by 31P-nuclear magnetic resonance (NMR) spectroscopy. NMR spectra were obtained during rest and electrical stimulation of the muscle in the anaesthetized animal at 33 recording sessions. Electrical stimulation was applied by a pair of copper wires placed separately with hypodermic needles into the muscle. NMR spectra were acquired with a 2 x 3 cm, double-turn, copper transmit/receive coil. Sixteen spectra were averaged over 30 s to obtain averaged spectra continuously during a 30-min recording. The spectra were processed automatically using a non-linear 'least-squares' fitting program on the spectrometer. A Lorentzian line shape was assumed for the peaks, and values of peak height, area and chemical shifts were generated. Each averaged spectrum consisted of five peaks: inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to ATP. Data were analysed as to absolute changes in Pi and PCr, in the ratio of Pi/PCr, and the shift of Pi to PCr to estimate pH. Several protocols were used in which ranges of frequency, intensity and duration of electrical stimulation were tested. The protocol for detailed studies involved stimulating the muscle twice at 5 Hz for 3 min separated by a 3-min rest period, then stimulating twice at 50 Hz for 3 min separated by a rest period. During contraction of the muscle, there was a significant increase in the Pi/PCr ratio (p < 0.05) as compared to the resting level. The ratio reached a plateau over a 3-min contraction using 5-Hz stimulation, then increased significantly more with the 50-Hz stimulation but decayed during the 3 min. Sustained stimulation with 50 Hz for 15-45 min evoked an initial sharp change in Pi/PCr, which then reached a steady plateau that remained over the entire stimulation. These findings indicate that the rabbit masseter muscle is relatively fatigue resistant in maintaining a steady-state equilibrium in the relation of Pi to PCr.

Relationships between alkali light-chain complement and myosin heavy-chain isoforms in single fast-twitch fibers of rat and rabbit

The present study compares the alkali myosin light chain (LC) complement of the fast fiber types IIB, IID and IIA in single fibers from rat muscle, as well as in type IID and type IIA fibers from rabbit muscle. Single fibers were classified according to their electrophoretically determined myosin heavy chain (HC) isoforms, HCIIb, HCIId, and HCIIa. Alkali myosin light chains were analysed by densitometric evaluation of two-dimensional electrophoresis performed on extracts from the same fibers. On the average, the fraction of LC3f, i.e. LC3f/(LC1f+LC3f), was highest in type IIB fibers and lowest in type IIA fibers. Type IID fibers occupied an intermediate position. Also in the rabbit, type IID fibers displayed a higher fraction of LC3f than type IIA fibers. Large scattering of the LC3f fraction in IIB, IID, and IIA fibers indicated that each fiber type is composed of fibers identical with regard to their specific myosin heavy chain complement, but heterogeneous with regard to their fast alkali light chain composition and the resulting light-chain-based isomyosins. It is suggested that the variable proportions of the two alkali light chains in the three fast fiber populations serve as a fine tuning of contractile velocities within the ranges determined by the three fast myosin heavy-chain isoforms.

Myosin heavy-chain-based isomyosins in developing, adult fast-twitch and slow-twitch muscles

A modified method of electrophoresis under nondenaturing conditions made it possible to separate rat muscle extracts of defined myosin heavy chain (HC) and light chain (LC) composition into subsets of developmental, fast and slow myosin heavy-chain-based isomyosins. The fastest migrating isomyosins were the neonatal isomyosins (nM1, nM2, nM3), followed by the slightly slower migrating embryonic isomyosins (eM1, eM2, eM3, eM4). Of the nine adult fast isomyosins, the HCIIb-based isomyosins (FM1b, FM2b, FM3b) were the fastest migrating. These were followed by the HCIId-based isomyosins (FM1d, FM2d, FM3d). The HCIIa-based isomyosins (FM1a, FM2a, FM3a) were the slowest. Our results suggest that FM3a is identical with the so-called intermediate isomyosin (IM) described in the literature. The slow myosin heavy-chain-based isomyosins (SM1, SM2, SM3) migrated far behind the fast isomyosins. Whereas the gross electrophoretic mobilities of each of these isomyosin triplets is determined by the specific heavy chain complement, the different mobilities of the bands within each triplet result from different alkali light chain combinations. Thus, the fastest triplet bands of the neonatal (nM1) and adult fast isomyosins (FM1b, FM1d, FM1a) represent the LC3f homodimers, the slowest (nM3, FM3b, FM3d, FM3a) the LC1f homodimers, and the intermediate bands (nM2, FM2b, FM2d, FM2a) the LC1f/LC3f heterodimers. Different proportions of the adult fast isomyosin triplet bands indicate that the affinity for LC3f decreases in the order HCIIb, HCIId, HCIIa. The three slow isomyosins represent LC1sa (SM1) and LC1sb (SM3) homodimers and a LC1sa/LC1sb heterodimer (SM2). Circumstantial evidence suggests an inverse order in rabbit muscle where SM1 and SM3 most likely represent LC1sb and LC1sa homodimers, respectively.

An animal model for human masseter muscle: histochemical characterization of mouse, rat, rabbit, cat, dog, pig, and cow masseter muscle

The masseter muscle of several animal species was investigated by use of a histochemical method for the demonstration of acid-stable and alkali-stable myosin adenosine triphosphatase (ATPase). The following subdivisions of fiber types were used: Type I fibers show weak ATPase activity at pH 9.4, type IM fibers react moderately, and type II fibers react strongly. Rat and mouse masseter muscles contained type II fibers only, as did some rabbit masseter muscles, whereas other rabbit masseter muscles possessed equal amounts of type I and II fibers. Cat and dog masseter muscles possessed both type II and I fibers, with type II predominating. Cow masseter muscle consisted mainly of type I fibers, although some cow masseter muscles contained a very small number of type II fibers. Pig masseter muscle had both type I, II, and IM fibers. One of the characteristics of human masseter muscle is type IM fibers, which are rarely seen in muscles other than the masticatory muscles. Therefore, pig masseter muscle might be a suitable animal model for experimental studies, such as an investigation of the distribution and diameter of fiber types in the masticatory muscles before and after orthognathic surgery.

Characterization of monoclonal antibodies to human platelet myosin that recognize highly conserved epitopes within the 50 kDa fragment of myosin subfragment-1

Three monoclonal antibodies directed against human platelet myosin heavy chains (MCH) that recognize homologous sequences contained within the functionally active subfragment-1, in platelet and rabbit skeletal muscle myosin were studied. These antibodies are distinguished by their affinities to different myosins and their differential effect on various ATPase activities. Epitope mapping was accomplished by analyzing antibody binding to proteolytic peptides of myosin head subfragment-1 under various experimental conditions. The epitopes recognized by these anti-human platelet MHC monoclonal antibodies reside within a small region of the 50 kDa fragment, beginning 9 kDa from its C-terminus and extending a stretch of 6 kDa towards the N-terminus. These epitopes lie between residues 535-586, and are contained within a highly conserved area of myosin heavy chain.

A possible role for myosin light chain 1 slow of bovine muscle

Myosin light chain 1 slow (LC1slow) was purified from the bovine slow-twitch muscle masseter and used as an antigen in chicken. The antibody fraction from egg yolk was employed to show the distribution of LC1slow in samples of diverse bovine muscles. Adjacent cryosections were cut. One section was stained for myofibrillar adenosine triphosphatase (ATPase) activity and the other was challenged with the antibody fraction, followed by a fluorescently-labelled secondary antibody. The ATPase section (viewed with conventional optical microscopy) displayed fast and slow fibres, whereas the antibody-labelled section (viewed with epifluorescent optics) showed which of the slow fibres contained LC1slow. In some of the muscles examined, the slow fibres contained variable amounts of LC1slow. In other muscles, such as those involved in chewing, the slow fibres were all rich in LC1slow. Slow fibres in yet other muscles, for example deep hip muscles and an extraocular muscle, were devoid of LC1slow. Based on the probable functions of the muscles examined, the hypothesis is advanced that LC1slow is associated with movement, its absence from slow fibres signifying a postural role for those slow fibres. The present work also documents the LC1slow content and muscle fibre composition of 14 hind leg muscles of a single ox. The results suggest postural and locomotory roles for certain muscles.

Experimental studies on masseter muscle grafts in rats: comparison of donor muscles

Experimental studies were performed in rats to explore the feasibility of using limb muscles to replace masticatory masseter muscles. The results showed that if several small fusiform muscles such as the extensor digitorum longus (EDL), soleus (SOL) and plantaris (PLT) were used for the grafting, the graft regenerated and matured fully in the masseter bed, although the weights were lower than those of contralateral control masseter muscles. If a multipennate muscle such as the gastrocnemius was used for the grafting, the graft also regenerated but later degenerated due to the lack of innervation. This is probably due to the excessive amount of connective tissue in the multipennate muscle. The connective tissue proliferates faster than the regeneration of muscle fibres and, thus, renders the innervation of muscle fibres difficult. Other factors pertinent to the possible clinical application of muscle transplantation in oral region are discussed.

Variations in contractile properties of rabbit single muscle fibres in relation to troponin T isoforms and myosin light chains

1. The maximal velocity of shortening (Vmax), tension-pCa relationships and the contractile and regulatory protein composition were determined in single, chemically skinned fibres from adult rabbit plantaris muscles. 2. Three groups of fibres were identified based on their protein compositions. One group had exclusively the slow-type myosin heavy chain (MHC) and myosin light chains (LC) and had low velocities. Another group of fibres had mixtures of fast-type and slow-type MHCs and LCs and had intermediate shortening velocities. The third group of fibres had fast-type myosin heavy and light chains and high velocities. 3. The low-velocity fibres had a mean velocity (+/- S.E.M.) of 0.86 +/- 0.03 muscle lengths/s (ML/s) at 15 degrees C. The remaining fibres formed a continuum with respect to Vmax from 1.37 to 3.94 ML/s. These results indicate that a much greater diversity exists among single fibres from adult mammalian skeletal muscle than previously recognized. The intermediate- and high-velocity fibres formed a continuum (from slow to fast) with respect to the amount of myosin light chain 3 (LC3). That is, Vmax increased with the relative LC3 content in single fibres in the intermediate- and high-velocity groups in a quantitative, statistically significant manner. 4. Three isoforms of fast-type troponin T were identified among the intermediate- and high-velocity fibres. These fibres also contained fast-type troponin C and troponin I. As was the case with the relative LC3 content, these fibres also formed a continuum with respect to the relative proportions of the three isoforms of fast-type troponin T. It appears that different isoforms of troponin T are responsible for a slightly higher Ca2+ sensitivity of tension development in the high-velocity fibres compared to the intermediate fibres. The continuum in troponin T isoform composition paralleled an increase in Vmax among these fibres. 5. The low-velocity fibres had the highest Ca2+ sensitivity of the three groups and had exclusively the slow-type isoforms of the regulatory proteins in the troponin complex. 6. The co-ordinated variations in troponin T and LC3 compositions among the intermediate- and high-velocity fibres are discussed as a possible means for the further differentiation of the contractile properties of the fibres in these two groups, beyond that provided by myosin heavy chain isoforms alone.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat posterior temporalis muscle during postnatal development

Changes in myosin gene expression during the postnatal development of the homogeneously superfast kitten posterior temporalis muscle were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were polyclonals directed against the heavy chains of superfast and foetal myosins and monoclonals against the heavy chains of slow and fast myosins. The fibres of the posterior temporalis in the newborn kitten stained almost uniformly with the anti-foetal myosin antibody and the largest of these fibres stained strongly for superfast myosin. A subpopulation of fibres staining for superfast myosin also stained lightly for slow myosin. These slow staining fibres were evenly distributed in the centres of muscle fibre bundles, reminiscent of primary fibres in limb fast muscle. During subsequent development, slow myosin staining disappeared and superfast myosin replaced foetal myosin so that by 50 days the muscle was virtually homogeneously superfast as in the adult. Fast myosin was never expressed at any stage. It is proposed that fibres staining transiently for slow myosin are superfast primary fibres which are homologous to fast primary fibres recently described in regions of limb muscles devoid of slow fibres in the matured animal. Other jaw-closing muscles have significant populations of slow fibres in the mature animal and it is postulated that there exists in these muscles a second class of jaw primary fibres, the slow primary fibres, in which slow myosin synthesis would be sustained in the adult. It is suggested that the myogenic cells of jaw-closing and limb muscles are of two distinct types preprogrammed to express different muscle genes.

Exercise training induces transitions of myosin isoform subunits within histochemically typed human muscle fibres

Fibre type composition based on histochemical myosin ATPase reaction was studied in cross sections of biopsies from the vastus lateralis muscle of men. In addition, protein composition as well as peptide patterns of isolated myosin heavy chains were examined in batches of individually classified fibres from the same biopsies. High intensity endurance training during 8 weeks induces significant decreases by 31-70% of the type IIB fibre population in 3 of 4 subjects (in one case no change was observed). These decreases were offset by corresponding increases in either type I or type IIA fibres with the type IIC fibres remaining always below 3%. A total of 13 professional cyclists with training periods over several years have a 20 times lower content of type IIB fibres than 4 sedentary controls and a concomitant high content of 80% of type I fibres. The content of type I and type IIB fibres of 8 sprinter athletes did almost not differ from that of controls. Thus the type IIB fibres respond most sensitively with a decrease to aerobic endurance training. Since both type IIA and IIB fibres were identical in protein composition containing the same fast variety of myosin light chains and heavy chains as well as troponin-I, their interconversion could not be seen at the molecular level. However, the slow variety of myosin light chains and of troponin-I started accumulating after 8 weeks of training in type IIA fibres. Furthermore, the myosin heavy chain isoform started shifting by producing new peptide patterns that resemble the digestion pattern of slow myosin heavy chains in fibres which still classified as type IIA. These changes on the molecular level in type IIA fibres mark the beginning of their transition over the intermediate and variable type IIC fibres, towards the slow type I fibre.

Fractional synthesis rates in vivo of skeletal-muscle myosin isoenzymes

The synthesis rates of different myosin isoenzymes in a single muscle, and of the same isoenzymes in different muscles (soleus, masseter and plantaris), were measured. The rate of total protein synthesis was significantly higher in the soleus [greater than 95% slow myosin (SM)] than in the plantaris [greater than 95% fast myosin (FM)]. Two fast isoenzymes, FM2 and FM3, were synthesized at different rates in the masseter, and SM was synthesized at a faster rate than FM. Intermediate myosin had a synthesis rate similar to that of FM. There was a small but significant difference between the synthesis rates of the SM isoenzymes of the soleus and masseter muscles. FM3 was synthesized faster in the masseter than in the plantaris, whereas FM2 was synthesized faster in the plantaris than in the masseter.

Contractile properties and myosin isoenzymes of various kinds of Xenopus twitch muscle fibres

Single twitch muscle fibres have been isolated from various parts of the iliofibularis muscle of Xenopus laevis. After measurements of their isotonic contractile properties, myosin extraction was performed on individual fibres and the extract analysed by various forms of gel electrophoresis. In agreement with previous results three major fibre types, types 1, 2, and 3 could be discerned. Both mechanical data and native isomyosin patterns indicated a further subdivision of types 1 and 2 into subtypes (1n, 1s and 2f, 2n, respectively). Transitional forms between 2n and type 3 were also identified. Types 1 and 2 had the same kinds of light chains (LC1f, LC2, LC3f), but different heavy chains (HC) as observed on 7% SDS gels. Types 3 lacked LC3 and had a more slowly migrating LC1 (LC1s); their HC migration velocity was indistinguishable from that of type 2 HC. A comparison was made between LC1/LC3 ratio and contractile parameters for nine type 1n fibres and six type 2n fibres. No clear correlation was observed between light chain proportions on the one hand and force per cross-sectional area or shortening velocity on the other. It is concluded that myosin heavy chain composition is the major determinant for contractile performance in Xenopus skeletal muscle fibres.

The multiplicity of combinations of myosin light chains and heavy chains in histochemically typed single fibres. Rabbit soleus muscle

1. Six adult rabbit soleus muscles were analysed by isolating histochemically identified fibre pieces from freeze-dried serial cross-sections. 2. By the use of this method, four fibre types (I, IC, IIC and IIA) were identified and analysed micro-electrophoretically. 3. Type I fibres contained the slow myosin heavy chain HCI and the slow myosin light chains LC1s and LC2s. 4. Type IIA fibres contained the fast myosin HCIIa with the fast light chains and, in addition, either LC1s or both LC1s and LC2s. 5. The C fibres (IC and IIC) represented intermediate populations between types I and IIC (IC) and between IC and IIA (IIC). They contained varied ratios of HCI/HCIIa with both sets of fast and slow light chains. With regard to myosin composition and isoforms of other myofibrillar proteins (M- and C-proteins, alpha-tropomyosin, troponin I), IC fibres resembled type I and IIC fibres resembled type IIA. 6. The presence of various myosin light and heavy chains within a specific fibre suggests a multiplicity of isomyosins. Without consideration of LC1sa and LC1sb differences, at least 54 possible isomyosins can be derived: type I fibres contain one isomyosin, types IC and IIC 54 possible isomyosins, and type IIA up to 18.

The developmental program of fast myosin heavy chain expression in avian skeletal muscles

We have examined the types of fast myosin heavy chains (MHCs) expressed in a number of different developing chicken skeletal muscles by combining peptide mapping and immunoblotting to identify fast MHC-specific peptides among the total mixture of MHC digestion products. Using this technique, we have identified three different fast MHC patterns among the different fast and mixed (i.e., fast and slow) fiber type muscles of the adult. While the different muscles all underwent sequential changes in fast MHC isoform expression during their development, the exact sequence of these changes and the isoform patterns expressed varied from muscle to muscle. During late embryonic or fetal development, all muscles expressed a similar fast MHC pattern (designated here as the fetal pattern) which was replaced shortly after hatching with a different fast MHC pattern (the neonatal pattern). During the transition from the neonatal to the adult state that occurred sometime in the first year after hatching, many of the muscles underwent additional changes in fast MHC isoform expression. In muscles such as the pectoralis major and pectoralis minor, a new fast MHC isoform pattern was seen in the adult so that the developmental program of isoform switching in these muscles involved the sequential appearance of distinct fetal, neonatal, and adult fast MHCs. Other muscles, such as the sartorius and posterior latissimus dorsi, underwent a qualitatively different program of isoform switching and expressed as an adult a fast MHC pattern that was indistinguishable from that expressed during fetal development. Finally, in some muscles, such as the superficial biceps, no change in isoform pattern was detected during the neonatal to adult transition--in these muscles, expression of the neonatal MHC isoform pattern apparently persisted into the adult state. These data indicate that no single scheme or program of fast MHC isoform switching can describe all the developmental changes that occur in fast MHC isoform expression in the chicken and that at least three different programs of isoform switching and expression can be identified.

Adaptations in the temporalis muscles of rabbits after masseter muscle removal

Masseter muscles were surgically removed in six young female rabbits so that we could study adaptations of the superficial temporalis muscles (ST) to increased functional requirements. Eight weeks following surgery, we used morphological measurements, histochemistry, contractile properties in situ, and occlusal force in vivo to compare the muscles in the experimental animals and six control rabbits. Analysis of the results demonstrated a decrease in fatigability of ST after masseter myectomy. Incisal occlusal force decreased by 65% during the first two weeks, and no recovery was observed during the following six weeks. At eight weeks post-surgery, the mass, twitch tensions, and tetanic tensions of ST were not significantly different from those of the controls. An increase in the percent of the cross-sectional area composed of fast fatigue-resistant fibers, a slower time-to-peak twitch tension, and a decrease in fatigability suggest an increase in oxidative metabolism. Analysis of these results suggests that muscles used for highly repetitious activities with submaximal loadings adapt to increased functional requirements by increasing fatigue-resistant properties.

Velocity of shortening and myosin isozymes in two types of rabbit fast-twitch muscle fibers

The fast-twitch tibialis anterior muscle of the rabbit was stimulated (10 Hz, 8 h/day for 7 wk) to cause complete transformation of the fibers from type IIb to type IIa. The velocity of unloaded shortening of permeabilized single fiber segments dissected from control and chronically stimulated tibialis anterior muscles was measured by the slack test at 20 degrees C. The myosin isozymes in these segments were separated on pyrophosphate-containing polyacrylamide gels. Peptide mapping of the myosin chain was performed on the myosin bands cut from the gels. The velocity of unloaded shortening of the IIb fibers was significantly higher (2.50 +/- 0.09 fiber length/s; n = 6) than that of the IIa fibers (1.33 +/- 0.08 fiber lengths/s; n = 6). The two groups of fibers differed with respect to their alkali light chain complement, as assessed by nondenaturing gel analyses, and with respect to their myosin heavy chain complement, as demonstrated by peptide mapping. Thus two groups of fast-twitch muscle fibers that contain distinguishable myosin isozyme contents differ in their velocities of unloaded shortening by a factor of two.

Changes in skeletal-muscle myosin isoenzymes with hypertrophy and exercise

The patterns of myosin isoenzymes in fast- and slow-twitch muscles of the rat hindlimb were studied, by pyrophosphate/polyacrylamide-gel electrophoresis, with hypertrophy (induced by synergist removal) and with spontaneous running exercise of 4 and 11 weeks duration. At 11 weeks, changes with hypertrophy in the slow-twitch soleus, composed of greater than 95% SM2 (slow myosin 2) in normal muscles, were minor, and consisted of an increase in the SM1 and SM1', and a loss of intermediate myosin (IM), an isoenzyme characteristic of Type IIa fibres [Fitzsimons & Hoh (1983) J. Physiol. (London) 343, 539-550]. The changes were dramatic, however, in the fast-twitch plantaris muscle. There was a 3-fold increase in the proportion of SM. In addition, IM became the predominant isoenzyme in the profile of hypertrophied plantaris by 4 weeks. These increases were balanced by decreases in the proportion of FM2 (fast myosin 2), with FM1 completely absent from the profile at 11 weeks. The changes in the plantaris with exercise were similar in direction but not as extensive as those with hypertrophy, and FM1 remained present at control levels throughout the study. When hypertrophy and exercise were combined, the increase in slow myosin was equal to the sum of the increases with each treatment alone. Changes at 4 weeks were intermediate between those of control and 11-week muscles. Peptide mapping of individual myosin isoenzymes showed that the heavy chains of IM were different from either fast or slow heavy chains. Furthermore, IM was found to be composed of a mixture of fast and slow light chains. These changes suggest that a transformation of myosin from fast to slow isoforms was in progress in the plantaris in response to hypertrophy, via a Type-IIa-myosin (IM) intermediate stage, a phenomenon similar to that occurring in chronically stimulated fast muscles during fast-to-slow transformation [Brown, Salmons & Whalen (1983) J. Biol. Chem. 258, 14686-14692].

Comparison of myosins from the masseter muscle of adult rat, mouse and guinea-pig. Persistence of neonatal-type isoforms in the murine muscle

Adult rat, mouse, and guinea-pig masseter muscles display distinct myosin electrophoretic patterns. The rat muscle contains four main forms which by reference to the myosins of the IIB tensor fasciae latae, of the IIA mylohyoid, and of the red and white portions of the sternomastoid muscles, correspond respectively to the intermediate-type and to the three fast-type isoforms. The mouse masseter muscle contains only three main myosins, the intermediate-type and two fast-type isoforms. The guinea-pig muscle also displays only three bands, whose assignment is, however, less straightforward than in the murine species; their electrophoretic mobilities are not strictly the same as those of their homologous forms in rat and mouse. Comparison with the myosins of the tensor fasciae latae and of the sternomastoid muscles of guinea-pig allows their identification as intermediate and fast-type myosins. In addition to these typical adult-type forms, adult murine masseter muscles are observed to contain between zero and 30% of neonatal-type myosins. The comparison of the developmental transitions of myosins in the rat masseter with those in the skeletal muscles of the same animal indicates a delay in the appearance of the adult as well as in the disappearance of the neonatal-type myosins in the masseter muscle. Both the variability in myosin types with the animal species and the atypical presence of neonatal forms in the murine adults suggest that myosin expression in the masseter muscle is subjected to unusual regulations.

Correlation between myofibrillar ATPase activity and myosin heavy chain composition in rabbit muscle fibers

Combined histochemical and biochemical analyses were performed on single fibers of rabbit soleus muscle. Histochemically, four fiber types (I, IC, IIC, IIA) were defined. Of these, types I and IIA were separate, histochemically homogeneous groups. A heterogeneous C fiber population exhibited a continuum of staining intensities between types I and IIA. Microelectrophoretic analyses of specific, histochemically defined fibers revealed that type I fibers contained exclusively HCI, whereas type IIA fibers contained only HCIIa. The C fibers were characterized by the coexistence of both heavy chains in varying ratios, type IC with a predominance of HCI and type IIC with a predominance of HCIIa. A direct correlation existed between the myosin heavy chain composition and the histochemical mATPase staining and was especially evident in the C fiber population with its variable HCI/HCIIa ratio. This correlation did not apply to the myosin light chain complement.

Neural control of phenotypic expression in mammalian muscle fibers

In this review, the present knowledge about the mechanisms involved in the control of the phenotypic expression of mammalian muscle fibers is summarized. There is a discussion as to how the activity imposed on the muscle fibers by the motoneuron finally induces in the muscle cells the expression of those genes that define its particular phenotype. The functional and molecular heterogeneity of skeletal muscle is thus defined by the existence of motor units with varied function, while the homogeneity of muscle fibers belonging to the same motor unit is yet another indication of the importance of activity in the control of gene expression of the mammalian muscle fiber.

Characterization of a monoclonal antibody to myosin specific for mammalian and human type II muscle fibers

We have characterized a monoclonal antibody (McAb), ALD-19, generated against slow myosin from chicken anterior latissimus dorsi (ALD) muscle for use in studies of human and animal muscle fiber types. This McAb bound selectively to the 200 kDa myosin heavy chain band in immunoblots against chicken, rat and human myosins and showed selective staining of A bands in the myofibrils. The reactivity of ALD-19 with various myosin types was quantitated by radioimmunoassays. Fiber type analysis revealed unexpected specificity of McAb ALD-19 for type II mammalian muscle fibers. This antibody should, therefore be useful for identification and quantification of normal type II fibers in human muscle biopsy specimens.